1. Field of the Invention
This invention relates to a device for the purification of an exhaust gas discharged from an internal engine for automobiles and the like.
2. Description of the Related Art
As a catalyst for the purification of an exhaust gas discharged from an internal engine for an automobile or the like, there are widely used catalysts capable of simultaneously conducting oxidation of carbon monoxide (CO) and hydrocarbon (HC) and reduction of nitrogen oxide (NOx). As disclosed in JP-B-58-20307, a greater part of these catalysts are formed by adding a noble metal such as palladium (Pd), platinum (Pt) or rhodium (Rh) as a catalyst component, and if necessary, a rare earth metal such as cerium (Ce), lanthanum (La) or the like or an oxide of a base metal such as nickel (Ni) or the like as a co-catalyst component to an alumina coating layer formed on a refractory carrier.
Such a catalyst is strongly influenced by a temperature of the exhaust gas and an air-fuel ratio set in the engine. That is, the exhaust gas temperature for developing the purification performance by the catalyst for the automobile is generally required to be not lower than 300° C., while the catalyst most effectively acts at the air-fuel ratio near to a theoretical air-fuel ratio (A/F=14.6) balancing the oxidation of HC and CO and the reduction of NOx. In the automobile provided with a device for the purification of exhaust gas using the conventional three-way catalyst, therefore, this device is disposed at a position effectively developing the effect of the three-way catalyst and also a feedback control is carried out by detecting an oxygen concentration in the exhaust gas so as to hold the air-fuel mixture at approximately theoretical air-fuel ratio.
If the conventional three-way catalyst is arranged just after an exhaust manifold, the catalyst activity is low immediately after the start of the engine in which the exhaust gas temperature is low (not higher than 300° C.), so that there is caused a problem that a great amount of HC discharged just after the engine start (cold start) are discharged as they are without purification.
As the exhaust gas purifying device for solving the above problem, an HC trapper enclosing an adsorbent for adsorbing cold HC is arranged on an upstream side of a catalyst convertor for the exhaust gas as proposed in JP-A-2-135126 and JP-A-3-141816.
In the exhaust gas purifying device disclosed in JP-A-2-135126, however, there are problems that (1) since the catalyst component is impregnated at the downstream side of the adsorbent, HC drops off from the adsorbent before the catalyst reaches to its active temperature; and (2) since a solution of a catalyst metal is impregnated in zeolite, the durability of the catalyst component is poor. On the other hand, the exhaust gas purifying device disclosed in JP-A-3-141816 has a problem that (3) the control of dropping off the adsorbed HC is carried out by using a temperature sensor, a by-pass pipe, a control device and the like, so that the layout of exhaust system becomes complicated and the reliability is poor and impractical.